1. Field of the Invention
The present invention relates generally to semiconductor quantum dot structures and devices.
2. Description of the Related Art
A semiconductor quantum dot is a structure having energy barriers that provide quantum confinement of electrons and holes in three dimensions. Because of these properties, there is interest in using quantum dot structures in a variety of electronic and optoelectronic devices. For example, studies indicate that quantum dot lasers can potentially perform better than conventional quantum well lasers in many respects. A quantum dot laser can have a lower fill factor (i.e., volume of material to be pumped) and/or an improved density of states function compared to a quantum well laser. Studies also indicate that the threshold current of a semiconductor laser may be improved by using quantum dot active regions, due to the smaller volume of material and reduced number of states.
Quantum dot lasers emitting light in the 1.3 to 1.6 micron wavelength range are particularly interesting for fiber optic communication systems. In particular, there has been interest in using InAs quantum dots as the active region for semiconductor lasers. Conventional approaches to creating InAs quantum dots are typically based on spontaneous processes that occur when growing strained on GaAs or InP substrates. Unfortunately, these conventional approaches have significant drawbacks that limit the usefulness of the resulting quantum dot structures, particularly in the 1.3 to 1.6 micron wavelength range.
For example, one important challenge is controlling the shape of the quantum dot. Usually, a high degree of symmetry in the shape of the quantum dot is desirable. For instance, a spherical quantum dot results in isotropic optical properties that can be important in certain optoelectronic applications. However, conventional approaches have not been able to produce symmetric InAs quantum dots on an InP substrate in a manner that is useful for optoelectronic devices. Instead, the resulting quantum dots are elongated in shape and generally referred to as quantum dashes. The resulting devices are highly anisotropic and the lack of symmetry limits the device performance. Attempts to create more symmetric InAs quantum dots have resulted in inadequate device properties, such as high threshold current, low modal gain and strong temperature dependence. These deficiencies are believed to be caused by a failure to achieve the desired shape and/or density for the InAs quantum dots.
Thus, there is a need to create improved quantum dot structures.